A stent is typically a hollow, generally cylindrical device that is deployed in a body lumen from a radially contracted configuration into a radially expanded configuration, which allows it to contact and support a vessel wall. A plastically deformable stent may be implanted during an angioplasty procedure by using a balloon catheter bearing a compressed or “crimped” stent, which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a support for the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally, and positioned at a desired location by means of the balloon catheter.
Stents may be formed from wire(s) or strip(s) of material, may be cut from a tube, or may be cut from a sheet of material and then rolled into a tube-like structure. Magnesium alloys are being investigated as a material for a bioabsorbable stent due to their biocompatibility and ability to degrade in vivo. A three to six month period of mechanical integrity is desired, with complete degradation within twelve months. Magnesium alloys generally corrode too quickly to be used bare. Many methods that may be used to slow corrosion, such as anodization, have not been effective and are also typically work/time intensive processes.
Hydrothermal treatment is one option to slow corrosion of magnesium alloy stents. During the hydrothermal process, a hydrothermal conversion film (hydrothermal film) is formed on the surface of the metal sample and acts as a barrier to slow down the rate of corrosion of the metal sample. Although hydrothermal films have been used for various applications in the past for relatively large metal samples with simple geometries, hydrothermal films have not been used on complex geometries with high surface area to volume ratios, such as stents.
The hydrothermal films reported in the literature typically have large thicknesses and their corrosion prevention effectiveness has been demonstrated under static conditions. Hydrothermal films have not been considered to be applicable to stents, where a very thin layer of film is desired due to the small size of a stent, as well as the need to withstand bending and other dynamic conditions. Hydrothermal films are also not typically utilized for samples that must withstand corrosion while implanted in the human body. New coating technologies are needed to delay magnesium corrosion for up to three months, as well as to provide a more uniform corrosion throughout the stent.